1. Field of the Invention
This invention relates broadly to processes for manufacturing helically corrugated pipe gaskets.
2. Prior Art
In general, most applications of helically corrugated pipe require that the pipe be soil tight and convey liquids, such as water, without gross leakage, as this would cause undue setting or erosion of the soil surrounding the pipe. Furthermore, the pipe joints must also provide sufficient strength to maintain the proper alignment of the joined pipe sections throughout the life of the pipe. In joining adjacent sections of helically corrugated, pipe, the prior art has found that the corrugations of the pipe sections to be joined and the corrugations of band-type couplings must be of substantially the same pitch, size and depth. Good examples of this earlier art can be found in: U.S. Campbell Pat. No. 3,239,254 issued Mar. 8, 1966 and entitled "Pipe Coupling," U.S. Wiley Pat. No. 3,298,721 issued Jan. 17, 1967 and entitled "Coupling Band for Joining Corrugated Pipes," U.S. Davis Pat. No. 3,315,991 issued Apr. 25, 1967 and entitled "Pipe Coupling;" and U.S. Campbell Pat. No. 3,708,187 issued Jan. 2, 1973 and entitled "Conduit Seal." However, the use of these coupling devices required that the corrugations of the abutting pipes be aligned so that the band can properly seat on the pipe section ends. This generally requires rotation of one pipe which is very time consuming and not practical, particularly in large diameter pipes. This and other problems, as more completely discussed in U.S. Boynton, et al Pat. No. 3,501,179 issued Mar. 17, 1970 and entitled "Helically Corrugated Spiral Pipe and Coupling Therefor," have led to attempts to rectify these problems. One of the earlier attempts is disclosed in U.S. Davis Pat. No. 3,315,991 issued Apr. 25, 1967 and entitled "Pipe Coupling" and is also discussed in the aforementioned U.S. Boynton, et al Pat. No. 3,501,179. In this disclosure, a pair of narrow, deformable elongated strips having a broad base are strapped to each pipe by a special metal band which, upon tightening, deforms the strips to match the pipe corrugations. The difficulty in such a design is the ability of the deformed narrow strips to form and maintain the necessary seal, particularly in pipes having corrugations with great depth, and in cases where settling of the pipes is likely to occur. For this reason, the most commonly used corrugated pipes, particularly where proper seals are necessary, is reformed pipe wherein the helical corrugations at the pipe ends have been reworked into annular corrugations. This pipe design is much easier to seal with various couplings such as that shown in the aforementioned U.S. Boynton, et al Pat. No. 3,501,179. The effectiveness of this arrangement, as attested by its wide industry usage, however, relies on a reworked pipe whose cost is substantially higher than the helically corrugated spiral pipe. In the Campbell Patent, a segmented sealing strip is disclosed having helically spiraled corrugations. However, leakage resulting from the segmented nature of the strips has deterred the commercial acceptance of this design.
Therefore, if a strip could be economically manufactured that could be applied to helically corrugated pipe in the field that did not leak when the tightening band is applied, it would be welcomed in the industry.